docs: def _compute_standard_dev(self, rup, imt, C):
          """
          Compute the the standard deviation in terms of magnitude
          described on page 744, eq. 4
          """
          sigma_mean = 0.
          if imt.name in "SA PGA":
              psi = -6.898E-3
          else:
              psi = -3.054E-5
          if rup.mag <= 6.5:
              sigma_mean = (C['c12'] * rup.mag) + C['c13']
          elif rup.mag > 6.5:
              sigma_mean = (psi * rup.mag) + C['c14']
          return sigma_mean